1. Field of the Invention
The present invention relates to a method for controlling a tunable filter to be utilized in wavelength or frequency division multiplexing systems or the like, an apparatus therefor and an optical communication network including such apparatus.
2. Related Background Art
In general, a tunable filter has a transmission coefficient for a wavelength of light to be input therein as shown in FIG. 1. When multiplexed optical signals of different wavelengths are input into such tunable filter, a signal of wavelength selected according to the relationship between wavelength and transmission coefficient of the tunable filter is filtered out. Normally, a signal of a single wavelength is taken out of the multiplexed signals of different wavelengths input into the tunable filter since a wavelength interval between adjacent signals of the multiplexed signals is wider than a half-value width (wavelength width at a transmission coefficient whose value is a half or 50% of that of a maximum transmission coefficient: see FIG. 1) of the tunable filter. The intensity of the selected or filtered signal is at its maximum when a center transmission or filtering wavelength of the tunable filter is coincident with that to be selected.
Usually, the center filtering wavelength of a tunable filter can be changed or tuned by controlling a current flow through a terminal thereof for changing its filtering wavelength. Therefore, if the relationship between a value of such current flow and the center filtering wavelength is already known, a signal of any desired wavelength can be filtered out by causing the current corresponding to the desired wavelength to flow through a filtering-wavelength changing terminal of the tunable filter.
FIG. 2 shows an example of prior art methods for controlling a tunable filter. In FIG. 2, a voltage-to-current (V-I) converter 201 and a constant current source 202 are connected to a tunable filter 203. In the prior art method, the relationship between a filtering wavelength control current and a filtering wavelength of the tunable filter 203 needs to be accurately measured in the first place. The measurement is conducted under a condition under which temperature is stably set to a predetermined value since the filtering wavelength control current-dependent filtering wavelength characteristic of the tunable filter 203 is greatly varied due to the change in temperature.
The relationship between the filtering wavelength of the tunable filter 203 and a wavelength selection control signal input to the converter 201 is thus determined from the above-mentioned measured value, considering a voltage-current converting characteristic of the converter 201. Further, a gain control current-to-gain characteristic of the tunable filter 203 is measured.
The operation of the prior art system is as follows. First, the temperature is set to the predetermined value at the time of measurement. Then, the gain control current is caused to flow into the tunable filter 203 through its gain control terminal by the current source 202 to set a gain of the tunable filter 203 to a given value. Further, the wavelength selection control signal is set to a voltage corresponding to a wavelength of any desired siganl. This control voltage is converted into a current by the voltage-to-current converter 201, and this current is caused to flow into the tunable filter 203 through its wavelength control terminal.
Thus, the filtering wavelength of the tunable filter 203 is set to the wavelength of the desired signal. When multiplexed signals of different wavelengths are input into the tunable filter 203, the signal of the desired wavelength is output pursuant to a transmission factor characteristic of the tunable filter 203 since the center filtering wavelength of the tunable filter 203 is coincident with that of the signal desired to be output.
In the prior art system, however, when the wavelength of an input signal or center filtering wavelength of the tunable filter 203 is changed, an amount of the signal light to be output is reduced. In the worst case, the desired signal will not be output at all. As a result, temperature stabilization circuits are required for stabilizing the wavelength of an input light emitted from a light source (i.e., stabilizing a laser diode for emitting a light signal) and the center filtering wavelength of the tunable filter 203. Further, a highly precise and stable voltage-to-current converting circuit 201 with little variance is required for accurately setting the value of current to be supplied to the wavelength control terminal of the tunable filter 203.
However, it is difficult to always maintain the center filtering wavelength of the tunable filter 203 at a constant value since techniques for realizing the converting circuit 201 mentioned above are hard to achieve.
Moreover, the current-to-wavelength characteristic of the tunable filter 203 must be accurately measured beforehand, so that the number of steps of a process for controlling the tunable filter 203 will be increased.
For solving the above-discussed problem, there may be a solution that a transmission band of a tunable filter is made wide enough to cope with wavelength fluctuations, but this solution inevitably restricts the possible multiplexing number of signals of different wavelengths.